1. Field of the Invention
The present invention relates to a flow rate measuring device suitable for measuring an intake air amount of an internal combustion engine, for example.
2. Description of the Related Art
According to a flow rate measuring device described in JP-A-2007-93422 (hereinafter referred to as Patent Document 1), a circuit mount portion is formed at the base end side of the flow rate measuring device disposed in a main channel, and a measuring channel is formed at the tip side of the flow rate measuring device. The measuring channel comprises a flow-in port, first, second, third and fourth bending portions, first, second, third, fourth and fifth channels and a flow-out port, and a flow rate detecting element 6 is disposed in the third channel. A part of measurement target fluid flowing through the main channel flows from the flow-in port into the measuring channel, flows along the surface of the flow rate detecting element, flows out from the flow-out port and then joins the measurement target fluid flowing through the main channel. A heater is provided to the flow rate detecting element, and supplied with current from a control circuit to generate heat. The heater is brought into contact with intake air flowing along the surface of the heater and cooled, whereby the flow rate detecting element detects the flow rate of the intake air as a resistance value variation corresponding to the temperature.
In the conventional flow rate measuring device disclosed in the Patent Document 1, the flow rate detecting element is disposed in the third channel between the second bending portion and the third bending portion. At the second bending portion, the flow direction of the measurement target fluid is changed by 90°, and a portion at which turbulence of flow occurs due to exfoliation of flow at the inner corner side and impinge of flow against the outer corner portion exists in the third channel. When the flow rate detecting element is disposed at the portion where the flow turbulence occurs, an output signal is affected by the flow turbulence and thus it is also fluctuated, so that the measuring precision is deteriorated.
Furthermore, in a thermal type flow rate measuring device, when flow is pulsated due to thermal response delay of a flow rate detector and a non-linear output characteristic, there occurs a lean error that the average flow rate detected by the flow rate measuring device is smaller than the true average flow rate occurs.
Furthermore, when pulse flow accompanied by counter flow occurs, the thermal type flow rate measuring device which has no counter flow detecting function detects the pulse flow as forward flow, so that a rich error that the average flow rate detected by the flow rate measuring device is larger than the true average flow rate occurs.
It has been known by persons skilled in the art to use a bypass channel in order to reduce these pulsation detection errors, and a bypass flow channel is adopted in the Patent Document 1 to reduce the pulsation detection error. That is, a pressure pulsation which is delayed with respect to the pressure pulsation at the flow-in port and the flow-out port of the bypass channel occurs in the bypass channel, and thus the average flow rate is increased, so that the lean error can be reduced. Furthermore, the bypass channel is designed to have a flow-channel structure into which the counter flow hardly flows, whereby the rich error can be reduced.
However, the effect of reducing the pulsation detecting error as described above is dependent on the arrangement position of the flow rate detecting element. However, no attention has been hitherto paid to the point in the conventional device, and thus the effect of reducing the pulsation detection error cannot be sufficiently obtained.